[BUG?] Add support to Qt6/Pyside

[amreo]

If I add parts to a multiple_tabs_command I get this error:

15:35:54  pyException: Traceback (most recent call last):
  File "/home/amreo/.local/share/FreeCAD/v1-1/Mod/LCInterlocking/./panel/treepanel.py", line 100, in getStandardButtons
    return int(QtGui.QDialogButtonBox.Ok | QtGui.QDialogButtonBox.Cancel)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
<class 'TypeError'>: int() argument must be a string, a bytes-like object or a real number, not 'StandardButton'
15:35:57  Traceback (most recent call last):
15:35:57    File "/home/amreo/.local/share/FreeCAD/v1-1/Mod/LCInterlocking/./panel/treepanel.py", line 182, in add_parts
15:35:57      self.force_selection(last_index)
15:35:57    File "/home/amreo/.local/share/FreeCAD/v1-1/Mod/LCInterlocking/./panel/treepanel.py", line 286, in force_selection
15:35:57      self.selection_model.select(index, QItemSelectionModel.ClearAndSelect | QItemSelectionModel.Rows)
15:35:57  TypeError: 'PySide6.QtCore.QItemSelectionModel.select' called with wrong argument types:
  PySide6.QtCore.QItemSelectionModel.select(NoneType, SelectionFlag)
Supported signatures:
  PySide6.QtCore.QItemSelectionModel.select(selection: PySide6.QtCore.QItemSelection, command: PySide6.QtCore.QItemSelectionModel.SelectionFlag, /)
  PySide6.QtCore.QItemSelectionModel.select(index: PySide6.QtCore.QModelIndex | PySide6.QtCore.QPersistentModelIndex, command: PySide6.QtCore.QItemSelectionModel.SelectionFlag, /)

I think that this error happens because LCInterlocking doesn't support Qt6/Pyside6, used in newer version of freecad.

OS: Manjaro Linux (GNOME/gnome/wayland)
Architecture: x86_64
Version: 1.1.0rc3.20260223 (Git shallow) AppImage
Build date: 2026/02/23 23:43:19
Build type: Release
Branch: (HEAD detached at adf7760)
Hash: adf77600e2526bf02804524f0d1991ab1c534abe
Python 3.11.14, Qt 6.8.3, Coin 4.0.3, Vtk 9.3.1, boost 1_86, Eigen3 3.4.0, PySide 6.8.3
shiboken 6.8.3, xerces-c 3.3.0, IfcOpenShell 0.8.4, OCC 7.8.1
Locale: Italian/Italy (it_IT)
Navigation Style/Orbit Style/Rotation Mode: Touchpad/Rounded Arcball/Window center
Stylesheet/Theme/QtStyle: FreeCAD.qss/FreeCAD Dark/
Logical DPI/Physical DPI/Pixel Ratio: 96/107.244/1
Installed mods: 
  * fasteners 0.5.44
  * LCInterlocking 1.5.1

This bug makes the workbench a little too buggy